COPENHAGEN 芒鈧珺次元官网网址� The United Nations and Congo must investigate after three bodies, including those of Swedish and American investigators with the United Nations, were found in central Congo, Sweden's prime minister said Wednesday.

Stefan Lofven said Sweden was "naturally ready to assist" in investigating the deaths of Swedish national Zaida Catalan, American Michael Sharp and their interpreter Betu Tshintela. Their bodies were found this week in a shallow grave.

They went missing March 12 along with driver Isaac Kabuayi and two motorbike drivers while looking into large-scale violence and alleged human rights violations by the Congolese army and local militia groups in Central Kasai province.

Their disappearance is the first time U.N. experts have been reported missing in Congo, Human Rights Watch has said, and it is the first recorded disappearance of international workers in the Kasai provinces.

Sharp, from western Pennsylvania, and Catalan were "killed senselessly," the U.S. ambassador to the U.N., Nikki Haley, said in a statement.

Congo government spokesman Lambert Mende late Tuesday said investigations will continue to seek other missing Congolese colleagues.

U.N. Secretary-General Antonio Guterres said late Tuesday the world body would conduct an inquiry into what happened to the two experts. He said the cause of their deaths hadn't yet been determined.

Parts of Congo, particularly the east, have experienced insecurity for decades, but violence in the Kasai provinces in central Congo represents a new expansion of tensions.

The Kamwina Nsapu militia has been fighting security forces since last year, with the violence increasing after government troops killed the militia's leader in August. More than 400 people have been killed and more than 200,000 displaced since then, according to the U.N.

Jan M. Olsen, The Associated Press